This invention relates to a faulted circuit indicator for monitoring the current in an alternating current power distribution circuit and providing a fault registration or signal only when abnormally high overcurrent in the circuit is sufficient to have operated a protection device upstream from the faulted circuit indicator.
Faulted circuit indicators are generally employed in a power distribution circuit to sense the current in a line conductor, such as an overhead uninsulated line or an underground insulated cable, which connects a power source to a load. When there is a fault in the circuit, for example an insulation breakdown in an underground cable, the line conductor may become grounded thereby causing overcurrent, namely high amplitude current above a predetermined threshold level, to flow through the line conductor. In response to that overcurrent, the faulted circuit indicator trips and registers or records the fact that a fault exists in the line conductor, or somewhere in the distribution circuit such as in a transformer, downstream from the faulted circuit indicator. The fault current or overcurrent normally causes a protection device, such as a fuse, to operate and interrupt the distribution circuit, thereby clearing and isolating the fault. Meanwhile, the fault registration (which may take the form of a blinking light) continues after the fuse blows to allow a lineman or service personnel to pinpoint the location of the problem.
Unfortunately, prior faulted circuit indicators are not always reliable since they are subject to providing false fault registrations in response to non-fault conditions, namely transients and temporary overloads causing overcurrents that exceed the trip setting of the indicator but do not result from actual or true faults and do not effect operation of the protection device and interruption of the distribution circuit. For example, when a distribution circuit is initially closed and thereby energized, transformer magnetizing inrush current may have a peak amplitude exceeding the threshold or trip current rating of the indicator. As another example, capacitor inrush/outrush current may momentarily be greater than the threshold level.
Various approaches have been taken to cure this deficiency of prior faulted current indicators, but the remedies have introduced other shortcomings. For example, a current reset feature has been incorporated in a faulted circuit indicator to automatically erase a false fault registration when continuing current exists at an amplitude above a predetermined minimum and at the main frequency of the power source feeding the distribution circuit, thereby evidencing normal load current. When the main frequency is 60 hertz (Hz), the minimum amplitude level is typically 3 amperes. Automatic reset does not always function properly, however, since the normal load current may actually be less than 3 amperes on a lightly loaded high voltage distribution circuit (where the line voltage is, for example, 25 KV) during periods of minimum load consumption. It is not possible to reduce the minimum reset current, required to erase the fault registration, due to the influence of adjacent phases. Moreover, with automatic reset faulted circuit closings and reclosings will result in false fault registrations from non-fault transients.
Faulted circuit indicators have also been designed that cannot be tripped by overcurrent for an initial time period (for example, 0.5 seconds) following the circuit closure or reclosure. This is called inrush restraint and neutralizes the deleterious effects of inrush currents, allowing them to time out and decay to a harmless value. The disadvantage of inrush restraint is that it also prevents registration from valid faults during the same initial time period. If a fault exists when a lineman first closes a circuit, he has no convenient way of locating the fault because the protection device will have operated and interrupted the circuit before the faulted circuit indicator will function.
The faulted circuit indicator of the present invention constitutes a substantial improvement and advancement over those previously developed in that error-free results are always obtained and a false registration never occurs. If at any time the monitored distribution circuit experiences a fault where the overcurrent is sufficient to trigger an upstream protection device to open the circuit, that fault will be registered, and this is true even though the fault may already exist at the time the circuit is closed or energized. On the other hand a non-fault, that results in overcurrent but not in the operation of the protection device, will not be registered, thereby always providing an absolutely reliable indication of the condition of the monitored circuit.